UI3 is a powerful, modern HTML5 web interface for Blue Iris. Since April 20, 2018, UI3 is Blue Iris's default web interface for non-IE browsers and is included with Blue Iris versions 4.7.3 and newer.
UI3 has a dedicated thread on the ipcamtalk forum, here:
https://ipcamtalk.com/threads/blue-iris-ui3.23528/
Be aware that most UI3 releases are developed against the latest Blue Iris version at the time of UI3 release. Compatibility with older Blue Iris versions is not maintained, so any time you update UI3 without being on the latest version of Blue Iris, you risk encountering broken features and other bugs.
If you wish to manually install a UI3 update, you can get it from the releases tab: https://github.com/bp2008/ui3/releases
To install, just extract everything to Blue Iris's www directory and overwrite all files. The default path to this folder is C:\Program Files\Blue Iris 5\www.
The last Blue Iris 4 release (4.8.6.3) shipped with UI3-70, but you can update to UI3-77 for a few improvements. Things will begin breaking if you update beyond UI3-77 on a Blue Iris 4.x installation.